1. Technical Field
The invention relates to an optical transmitter for performing communication by generating optical signals having a pulse shape from a light-emitting element; an optical communication system; and a method for adjusting the optical transmitter.
2. Description of the Related Art
In the optical communication, in order to assure transmission quality, it is necessary to maintain an average light properly in the optical transmission.
FIG. 7 shows examples of waveforms of output signals having a pulse shape emitted from a light-emitting element. FIG. 7A shows ideal waveforms. FIG. 7B shows waveforms in the case where an extinction ratio is lowered although an average light intensity of the output signals is the same as that in FIG. 7A.
The output signals having the pulse shape emitted from the light-emitting element take either an H level or an L level. As shown in FIG. 7A, an average value of a light intensity P1 corresponding to the H level and a light intensity P0 corresponding to the L level constitutes an average light intensity. As the average light intensity is larger, the optical signals are less susceptive to noise. However, it has been known that even through the average light intensity of the output signals is equal to or larger than a predetermined value, when an extinction ratio “re” of the output signals is lowered, the transmission state of communication deteriorates. Here, the extinction ratio “re” is expressed by the following formula,re=10×log(P1/P0)   (1)
FIG. 8 illustrates relationships between a driving current and a light intensity. As shown in FIG. 8, light-emitting characteristics of the light-emitting element includes a spontaneous emission region 5a where light intensity does not almost increase even if the driving current is increased and a stimulated emission region 5b where light intensity greatly increases with an increase in the driving current. Further, the light-emitting characteristics of the light-emitting element has such a temperature dependency that an inclination of the curve (slope efficiency η) in the simulated emission region 5b varies in response to temperatures. When the slope efficiency η varies, the extinction ratio “re” also varies. Further, the extinction ratio “re” also varies depending upon variations of respective elements in a driving circuit.
The slope efficiency η stands for a ratio ΔP/ΔI of an increase ΔP in the light intensity to an increase ΔI in the current in the stimulated emission region 5b shown in FIG. 8. As shown in FIG. 8, the slope efficiency η decreases when the temperature increases, and increases when the temperature decreases.